Biophysical limitation of leaf cell elongation in source-reduced barley

The biophysical basis of reduced leaf elongation rare in source-reduced barley (Hordeum vulgare L. cv Golf) was studied. Reduction in source strength was achieved by removing the blade of leaves I and 2 at the time leaf 3 had emerged 3.0-6.7 cm from the encircling sheath. Third leaves of source-reduced plants elongated at 10-36% lower velocities than those of control plants. Removal of source leaves had no significant effect on maximum relative elemental growth rates (REGRs) and the length of the elongation zone (42-46 mm) but caused a shift of high REGR towards the basal portion of the elongation zone. Cell turgor was similar between treatments in the zone of maximal REGR (16-24 mm from base), but was significantly lower in source-reduced plants in the distal part of the elongation zone, where REGR was also lower. Throughout the elongation zone, osmolality and growth-associated water potential gradients were significantly smaller in source-reduced plants; bulk concentrations of sugars (hexoses, sucrose) were also lower. However, even in control plants, sugars contributed little to bulk osmotic pressure (6-11%). The most likely biophysical limitation to leaf (cell) elongation in source-reduced barley was a reduction in turgor in the distal half of the elongation zone. It is proposed that in the proximal half, increase in average tissue hydraulic conductance enabled source-reduced plants to maintain turgor and REGR at control level, while spending less energy on solute transport.